1. Field of the Invention
The present invention relates to a laser heat treatment method and apparatus for forming a polycrystalline silicon film having an excellent crystalline property in order to fabricate a high-mobility thin film transistor, and a semiconductor device produced using such method and apparatus.
2. Description of the Background Art
At present, pixel portions of a liquid crystal panel produce an image by switching of thin film transistors that are formed from an amorphous or polycrystalline silicon film on a glass or synthetic quartz substrate. If a driver circuit (which is now typically independently mounted outside) for driving the pixel transistors can be simultaneously formed on the panel, significant advantages would be obtained in terms of production cost, reliability and the like of the liquid crystal panel. At present, however, since the silicon film forming an active layer of the transistor has a poor crystalline property, the thin film transistor has poor capability in terms of mobility and the like, thereby making it difficult to make an integrated circuit that requires high speed and functionality. Laser heat treatment is commonly conducted as a way to improve the crystalline property of the silicon film in order to fabricate a high-mobility thin film transistor.
The relationship between the crystalline property of the silicon film and the mobility of the thin film transistor is described as follows: in general, the silicon film resulting from the laser heat treatment is a polycrystalline film. Crystal defects are locally present at polycrystalline grain boundaries, which inhibit carrier movement in the active layer of the thin film transistor. Accordingly, in order to increase the mobility of the thin film transistor, it is only necessary to reduce the number of times for the carriers to move across the grain boundaries while moving in the active layer, and to reduce a crystal-defect density. The purpose of laser heat treatment is to form a polycrystalline silicon film having a large grain size and also having a small number of crystal defects at the crystal boundaries.
FIG. 11 is a diagram showing one example of a conventional laser heat treatment apparatus. In FIG. 11, a pulse laser source 501 is an excimer laser source (such as KrF (wavelength of 248 nm) and XeCl (wavelength of 308 nm)) that is a typical pulse laser source of a wavelength less than 350 nm for emitting ultraviolet light commonly used as heat treatment laser. Excimer laser light 502 is emitted from pulse laser source 501. A beam homogenizer 503 makes an intensity distribution of excimer laser light 502 uniform. A focusing optical system 504 focuses excimer laser light 502 An amorphous silicon from 505 is disposed so that it is subjected to the laser heat treatment. Amorphous silicon film 505 is formed on an underlying silicon oxide from 506 on a glass or quartz substrate 507.
Hereinafter, a conventional laser heat treatment method is described. Excimer laser light 502 emitted from pulse laser source 501 is directed through beam homogenizer 503 onto amorphous silicon film 505 by focusing optical system 504. Amorphous silicon film 505 is melted in the region irradiated with excimer laser light 502. Then, as the temperature is reduced, the melted silicon is crystallized to form a polycrystalline silicon film. Since silicon has an extremely high absorption coefficient for the excimer laser light, heat treatment can be efficiently conducted even to a thin silicon film. However, due to the excessively high absorption coefficient, the laser light will be absorbed by the time it advances about 10 nm from the surface. A melting process of amorphous silicon film 505 is shown in FIGS. 12A to 12D. FIG. 12A shows a state of silicon film 505 upon irradiation of the laser light in the direction shown by P; FIG. 12B shows a state obtained several tens of nanoseconds after the irradiation; FIG. 12C shows a state obtained several tens of nanoseconds after FIG. 12B; and FIG. 12D shows a state after the crystal growth. Upon irradiation of the laser, silicon film 505 has a melting-depth distribution and temperature distribution corresponding to a Gaussian beam profile 601 shown in FIG. 12A, and a melted portion 603 of the silicon film is produced Heat is generally conducted at a certain spreading angle. Therefore, as the melting depth is increased by the heat conduction, these distributions become broader as shown in FIG. 12B, resulting in the uniform distributions as shown in FIG. 12C Thus, melted portion 603 of the silicon film is formed. Accordingly, since there is no lateral temperature distribution, recrystallization proceeds in the vertical direction, whereby the resultant crystal grains 604 have a vertically elongated shape as shown in FIG. 12D. In other words, the crystal grain size is reduced in the direction of the plane in which the carriers move.
FIG. 13 shows dependence of the mobility (n-channel) of a MOS transistor on the irradiation energy density of laser light, wherein the MOS transistor has its active layer formed from the polycrystalline silicon film thus obtained. FIG. 13 shows a result with the use of a KrF excimer laser source as pulse laser source 501 (FIG. 11), and pulse duration is about 15 nsec (FWHN). In addition, silicon oxide film 506 and amorphous silicon film 505 have a thickness of 200 nm and 50 nm, respectively. Herein, a laser-irradiation area is defined as an area having an irradiation intensity that is 1/e2 times or more of the peak value, and the irradiation energy density was calculated from the radiant laser energy. As can be seen from FIG. 13, under the laser-heat-treatment conditions as described above, the maximum mobility of 80 cm2/Vs was obtained by setting the excimer-laser irradiation energy density to 230 mJ/cm2, and about 80 percent or more of the maximum mobility was obtained in the range of xc2x15 mJ/cm2 therefrom. However, such mobility is still insufficient to make a high-speed, high-functionality integrated circuit. Moreover, as shown in FIG. 13, the mobility is highly dependent on the irradiation energy density. Therefore, in introducing such a method in the production line, the produced transistors will have variation in their characteristics unless laser output and focusing capability of the optical system are highly strictly controlled. The reason for this can be considered as follows: since silicon has a high absorptance of the excimer laser light, a melting state thereof is varied with a slight change in the irradiation energy density, so that a recrystallization process is changed.
In terms of an enhancement of grain size of the polycrystalline silicon film, an attempt has been made in articles to conduct laser heat treatment with long-wavelength laser light of 350 nm or more (Reference 1 (Appl. Phys. Lett. 39, 1981, pp. 425-427), Reference 2 (Mat. Res. Soc. Symp. Proc., Vol. 4, 1982, pp. 523-528), and Reference 3 (Mat. Res. Soc. Symp. Proc., Vol. 358, 1995, pp. 915-920). Herein, a second harmonic of Nd:YAG laser (wavelength of 532 nm) is used as the long-wavelength laser light of 350 nm or more. In these reported examples, a beam profile at the irradiated position corresponds to an asymmetric Gaussian distribution. According to References 1 and 2, a recrystallization process of the laser heat treatment using the second harmonic of Nd:YAG laser is described as follows: description is herein given with reference to FIGS. 14A to 14D. As shown in FIG. 14A, when a focused laser beam 611 having Gaussian beam profile 601 is directed from focusing optical system 504 onto silicon film 505 in the direction shown by P, a temperature distribution 612 that is very close to the Gaussian distribution is produced within silicon film 505. Therefore, a melted portion 613 is formed in a melting state as shown in FIG. 14B. In a shallow portion C of the melting depth in FIG. 14B, a longitudinal temperature distribution is produced due to the heat loss mainly toward the substrate. As a result, a three-dimensional, isotropic crystal growth 614 occurs in the vertical direction as shown in FIG. 14C, whereby the recrystallized grain size is limited to a value as low as several hundreds of nanometers by the shallow melting depth. However, a portion D melted up to the interface with the substrate in FIG. 14B has a large temperature gradient in the lateral direction, resulting in a different recrystallization process 615 as shown in FIG. 14D. More specifically, the vertically grown crystal having a small grain size serves as seed crystal for lateral recrystallization toward the center having a high temperature. As a result, large crystal grains having a diameter of several micrometers are produced in the plane in which the carriers move.
In these reported examples, however, the axisymmetric Gaussian beam profile causes a significant problem. Since the profile is asymmetric at the irradiated position, crystal grains 616 grow radially as shown in FIG. 15. Accordingly, a MOS transistor having its active layer formed from this polycrystalline silicon film has such a structure as shown in FIG. 16. In FIG. 16, the transistor includes a source 701, a drain 702, a channel 704 interposed between source 701 and drain 702, and a gate 703 formed over channel 704. The active layer, which includes source 701, drain 702 and channel 704, is formed from the polycrystalline silicon film. Crystal grains 616 do not have the same orientation in channel 704 in which the carriers move. Since the carriers are scattered at the boundary planes of crystal grains 616 having different orientations, the carrier mobility is reduced. Moreover, since individual crystal grains have grown in a centrosymmetric manner, a gap, i.e., dislocation, a kind of crystal defects, is likely to be produced between the individual crystal grains, thereby increasing a crystal-defect density. Therefore, the polycrystalline silicon film resulting from the laser heat treatment has an extremely poor quality in terms of in-plane uniformity, and no successful thin film transistor has been reported so far.
Hereinafter, the relationship between the thickness of a silicon film and a MOS transistor is described. In general, as the silicon film forming the active layer becomes thinner, an s-factor as defined by dVG/d(logIDS) (where VG is a gate voltage, and IDS is a drain current) is reduced, whereby a threshold voltage is reduced. As a result, a driving voltage of the transistor is reduced, thereby achieving significant reduction in power consumption. Therefore, such a transistor is highly advantageously mounted in portable information terminals, a main application of the liquid crystal panels. However, since the silicon films used in References 1 and 2 are as thick as 0.2 to 1 xcexcm, they are not expected to practically function as a transistor due to their high threshold voltage and power consumption.
The laser heat treatment is usually conducted with a substrate being moved for large-area laser heat treatment. For the uniform quality of the resultant film, the substrate is commonly moved by a distance less than a beam width daring each interval of the laser-pulse irradiation, so that the laser is directed onto the same portion a plurality of times. According to Reference 3, it is desirable to direct the laser onto the same portion two hundred times or more. This is because of an increased X-ray diffraction peak intensity and reduced resistance of the silicon film after the laser heat treatment. Although Reference 3 does not mention the surface roughness, such a large number of times of irradiation generally produces significant surface roughness, and also causes partial ablation as well as removal of the silicon film from the substrate. In making a coplanar or staggered MOS transistor having its active layer formed from a polycrystalline silicon film, a gate oxide film is short-circuited if the surface roughness is large. Moreover, the MOS transistor itself cannot be formed if the silicon film is partially removed away.
In the conventional heat treatment using excimer laser, i.e., typical pulse laser having a wavelength of 350 nm or less, as a light source, a crystal grain size is small due to the vertical recrystallization growth, and the resultant thin film transistor has mobility as low as about 80 cm2/VS. Moreover, since the mobility is highly dependent on the irradiation energy density, constant mobility cannot be obtained, causing variation in characteristics of the resultant transistors.
On the other hand, in the conventional laser heat treatment using a second harmonic of Nd:YAG laser in order to enlarge crystal grains and thus increase the mobility of the thin film transistor, individual crystal grains do not have the same orientation due to the use of an axisymmetric Gaussian beam. Therefore, the resultant thin film transistor has low mobility and high crystal-defect density at the grain boundaries.
Moreover, in order to improve a crystal quality, as many as 200 shots or more of the laser is directed onto the same portion. Thus, the gate oxide film of the MOS transistor is short-circuited due to the large surface roughness, or the thin film transistor cannot be made due to ablation of the silicon film.
It is an object of the present invention to provide a laser heat treatment method for forming a thin film having an excellent crystalline property that is required to make a high-performance thin film transistor.
It is another object of the present invention to provide a productive, stable laser heat treatment method.
It is still another object of the present invention to provide a semiconductor device capable of operating at a high speed at low cost.
It is yet another object of the present invention to provide a laser heat treatment apparatus for conducting laser heat treatment to form a thin film having an excellent crystalline property.
A laser heat treatment method according to one aspect of the present invention comprises the steps of: forming a laser beam generated from a pulse laser source having a wavelength of 350 nm to 800 nm into a linear beam having a width and length; and directing the linear beam onto a film material formed on a substrate. According to this laser heat treatment method, a high-quality thin film having a large crystal grain size can be stably obtained.
In the laser heat treatment method of the present invention, the length of the linear beam is preferably ten times or more of the width thereof. In this case, lateral crystal growth can be reliably achieved, whereby a high-quality polycrystalline film can be obtained.
A laser heat treatment method according to another aspect of the present invention comprises the steps of: forming a laser beam generated from a pulse laser source having a wavelength of 350 nm to 800 nm into a linear beam having a width and length; and directing the linear beam onto a film material formed On a substrate, wherein the linear beam has an energy density gradient of 3 mJ/cm2/xcexcm or more in the width direction thereof. According to this laser heat treatment method, a higher-quality thin film having a larger crystal grain size can be stably obtained.
In the laser heat treatment method according to the another aspect of the present invention, an energy-density distribution in the width direction of the linear beam preferably has an approximately Gaussian profile. In this case, a post-anneal effect can be expected to be obtained.
In the laser heat treatment method according to the another aspect of the present invention, an energy-density distribution in the width direction of the linear beam preferably has an approximately top-flat profile. In this case, a peak value that causes ablation is suppressed, so that a gradient of the irradiation energy-density distribution can be increased.
In the laser heat treatment method according to the another aspect of the present invention, an energy-density distribution in the length direction of the linear beam preferably has an approximately top-flat profile of which standard deviation is 0.3 or less provided that an average intensity of the flat portion is 1. In this case, requirements regarding the performance of a beam-profile-forming optical system are reduced, whereby reduction in cost can be achieved.
In the laser heat treatment method of the present invention, the pulse laser source is preferably a harmonic of Q-switched oscillating solid-state laser using Nd ion- or Yb ion-doped crystal or glass as an excitation medium. In this case, efficient, stable heat treatment can be conducted.
In the laser heat treatment method of the present invention, the pulse laser source more preferably is any one of a second or third harmonic of Nd:YAG laser, a second or third harmonic of Nd:glass laser, a second or third harmonic of Nd:YLF laser, a second or third harmonic of Yb:YAG laser, or a second or third harmonic of Yb:glass laser. In this case, stable, productive heat treatment can be conducted at low cost.
In the laser heat treatment method of the present invention, the laser beam generated from the pulse laser source preferably has energy of 0.5 mJ or more per pulse. In this case, productive heat treatment can be conducted.
In the laser heat treatment method of the present invention, the laser beam generated from the pulse laser source preferably has pulse duration of less than 200 nsec. In this case, efficient heat treatment can be conducted.
In the laser heat treatment method of the present invention, an amorphous or polycrystalline silicon film is preferably used as the film material. In this case, heat treatment can be conducted with stable characteristics.
In the laser heat treatment method of the present invention, the amorphous or polycrystalline silicon film preferably has a thickness of less than 200 nm. In this case, a large grain size can be obtained, whereby excellent laser heat treatment can be achieved
In the laser heat treatment method of the present invention, the number of pulses of the pulse laser light directed onto the same portion of the amorphous or polycrystalline silicon film is preferably 100 pulses or less. In this case, a polycrystalline film having an excellent surface state can be obtained.
In the laser heat treatment method of the present invention, the irradiation energy density at a surface of the amorphous or polycrystalline silicon film is preferably in the range from 100 mJ/cm2 to 1,500 mJ/cm2. In this case, a polycrystalline film having an excellent surface state can be obtained.
A semiconductor device according to still another aspect of the present invention comprises a plurality of transistors each including an active layer. A laser beam generated from a pulse laser source having a wavelength of 350 nm to 800 nm is formed into a linear beam having a width and length, and the linear beam thus obtained is directed onto a film material formed on a substrate. Thus, the active layer is formed from the heat-treated film material on the substrate. At least one of the plurality of transistors, and preferably, a transistor operated at a higher frequency, has a drain current flowing in the direction approximately parallel to the width direction of the linear beam. In this case, a device operating at a high speed can be obtained at low cost.
In a laser heat treatment apparatus according to still another aspect of the present invention comprises a pulse laser source having a wavelength of 350 nm to 800 nm, and beam-forming optical means for forming a laser beam generated from the pulse laser source into a linear beam. By using this laser heat treatment apparatus, excellent heat treatment can be conducted in making a polycrystalline silicon film.
In the laser heat treatment apparatus of the present invention, the pulse laser source is preferably a harmonic of Q-switched oscillating solid-state laser using Nd ion- or Yb ion-doped crystal or glass as a laser excitation medium. In this case, a stable apparatus can be provided
In the laser heat treatment apparatus of the present invention, the pulse laser source more preferably is any one of a second or third harmonic of Nd:YAG laser, a second or third harmonic of Nd:glass laser, a second or third harmonic of Nd:YLF laser, a second or third harmonic of Yb:YAG laser, or a second or third harmonic of Yb:glass laser In this case, a stable, efficient apparatus can be provided at low cost.